1. Technical Field of the Invention
The present invention relates to an apparatus for estimating characteristics of transmission channels. The channel characteristics estimation apparatus of the present invention is suitable for receivers in a digital mobile communication system such as spread-spectrum CDMA.
2. Description of the Prior Art
Conventionally, the characteristics of transmission channels are estimated on the basis of pilot signals with prescribed patterns, in order to compensate distortions in the received data signals.
Such pilot signals are carried by the quadrature component, while the transmission data are carried by the in-phase component, as shown in FIG. 5. The receiver estimates the fluctuations in amplitudes and phases of the pilot signals in order to compensate the distortions in the received data.
The transmission data as shown in FIG. 5 is contained in a slot with a certain time interval. The pilot signal is also contained in the slot. Further, other signals are often inserted in a time slot which contains the pilot signal.
A conventional receiver is shown in FIG. 4, which receives the above-mentioned data and pilot signals.
The data and pilot signals received by antenna 1 are detected by matched filters 3 and 4, respectively. Each matched filter detects a correlation with each own pseudo-random sequence.
Channel characteristics estimation circuit 6 estimates characteristics of transmission channels on the basis of the output from matched filter 4, while delay circuit 5 delays the output from matched filter 3 by the time period which is required for the signal processing in channel characteristic estimation circuit 6.
Multiplier 7 compensates the transmission distortion by multiplying the output from delay circuit 5 by the output from channel characteristics estimation circuit 6. Further, a plurality of multipliers 7 for different timings are prepared, and the outputs from the plurality of multipliers 7 are added by adder 8. Here, each of multipliers are connected with matched filters 3 and 4, channel characteristics estimation circuit 6, and delay circuit 5.
An example of formula for estimating the channel characteristics is described in Reference 1 (THE TRANSACTION OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS B-11, Vol. J77-B-11, No. 11, pp. 628-640 (1994. November)).
The formula in Reference 1 approximates the channel characteristics in a linear form, by using a non-recursive filter on the basis of Ne samples of detected pilot signals X (nTb).
The formula 1 for obtaining tap coefficients Ci in Reference 1 is             C      i        =                                                      -              6                        ⁢            i                    +                      4            ⁢            Ne                    -          2                          Ne          ⁢                      (                          Ne              +              1                        )                              ⁢              xe2x80x83            ⁢              (                              i            =            O                    ,          …          ⁢                      xe2x80x83                    ,                      Ne            -            1                          )              ,
wherein Ne is a number of taps and Ci is a tap coefficient. Here, Ne is 150, and every sample has the same weight.
Another example of formula for estimating the channel characteristics is described in Reference 2 (THE TECHNICAL REPORT OF THE INSTITUTE OF ELECTRONICS, INFORMATION AND COMMUNICATION ENGINEERS, RCS97-163 (1997. November)).
Formula 2 for estimating the characteristics of a transmission channel in Reference 2 is                               ξ          ∼                l            ⁢              (        n        )              =                            1          _                          2          ⁢          L                    ⁢                                    ∑                          m              =                              -                L                                                          L            -            1                          ⁢                              β            m                    ⁢                                                    h                ⋀                            l                        ⁢                          (                              n                +                m                            )                                            ,
wherein 2L is a number of employed symbols, xcex2m is m-th weight coefficient, ĥl (n+m) is an estimated complex impulse response of (n+m)thsymbol in lth branch, and "xgr"l(n) is an estimated channel of nth symbol.
The channel estimation value calculated by the formula 2 is a weighted average on the basis of the weights xcex2m which becomes smaller, as the sample points go far from an observation timing. 160 samples around the point of notice are employed in Reference 2.
However, in the formula 1 for obtaining the characteristics of a transmission channel in the linear approximation, the sample points are not given weights depending upon the time difference from the observation timing.
Further, the formula merely calculates an average of the channel estimation value, although the sample points are given weights depending upon the time difference from the observation timing.
When the channel characteristics fluctuate rapidly, the linear approximation by formula 1 is more precise than the average by the formula 2. However, it is better to give less weight to the sample points which are far from the observation point, because the channel characteristics are time-dependent.
Unfortunately, it is not yet known how to vary the weights in formula 1 on the basis of the time difference from the observation timing.
Furthermore, memories sufficient to store 150 to 160 sample points and data processing capacity are required, because both the formula 1 and formula 2 employ as many samples in order to reduce an effect by noise.
Therefore, an object of the present invention is to provide a circuit which estimates the characteristics of a transmission channel in linear form, taking into consideration the time difference of sample points from observation timing.
Another object of the present invention is to reduce memory capacity and load of data processing for estimating the channel characteristics.
In the channel characteristics estimation apparatus of the present invention, an inclination and bias in the channel characteristics are calculated under 1st order approximation for every time slot. Then, the inclination parameter is processed by low pass filter, and the bias parameter is compensated on the basis of the compensated inclination parameter.
Such a low pass filtering makes it possible to estimate the channel characteristics on the basis of the time differences of the sample points. Further, above-mentioned calculation on the time slot basis makes it possible to reduce memory capacity and processing steps.
According to the present invention, a first order approximation of channel characteristics on the basis of the weights corresponding to the time differences of sample points from observation timing can be obtained, because the estimated inclination b (k) in the channel characteristics estimated slot by slot by the 1st order approximation is processed by a recursive low pass filter.
Further, according to the present invention, the memories and calculation steps required for estimating the channel characteristics are reduced, because the data are processed on the slot by slot basis.